1. Field of Invention
The present invention relates to a method of measuring aberration in an optical imaging system, such as a lithographic projection apparatus.
2. Discussion of Related Art
The patterning means here referred to should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this, it is necessary to increase the surface area of an IC and/or to decrease the size of the components. For the projection system, this means that both the image field and/or the resolution must be increased, so that increasingly smaller details, or line widths, can be imaged in a well-defined way in an increasingly large image field. This requires a projection system that must comply with very stringent quality requirements. Despite the great care with which such a projection system is designed and the very high accuracy with which the system is manufactured, such a system may still exhibit aberrations, such as spherical aberration, coma and astigmatism. In practice, the projection system (xe2x80x9clensxe2x80x9d) is thus not an ideal, diffraction-limited system, but an aberration-limited system. The aberrations are dependent on position in the image field and are an important source of variations in the imaged line widths occurring across the image field, as well as influencing the focus, exposure latitude and so on. They also cause field-dependent overlay errors between different mask structures and/or different illumination settings. The influence of aberrations becomes increasingly significant with the application of newer techniques, such as phase-shift masks or off-axis illumination, to enhance the resolving power of a lithographic projection apparatus.
A further problem is that the aberrations are not constant in modern lithographic projection systems. In order to minimise low-order aberrations, such as distortion, curvature of field, astigmatism, coma and spherical aberration, these projection systems generally comprise one or more movable elements. The wavelengths of the projection beam or the position of the mask table may be adjustable for the same purpose. When these adjusting facilities are used, other, smaller aberrations may be introduced. Moreover, since the intensity of the projection beam must be as large as possible, the components of the projection system are subject to ageing so that the aberrations may change during the lifetime of the apparatus. Moreover, reversible changes, e.g. as caused by lens heating, may temporarily change the aberrations.
Consequently there is a further problem of being able to measure the aberration reliably and accurately.
It is an object of the present invention to provide an improved method and apparatus for determining aberration of the projection system.
Accordingly, the present invention provides a method of determining aberration of an optical imaging system comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of substrate. Observations are made as a function of parameters of the projection apparatus, and from these the presence of different types of aberration can be quantified. The method comprises the step of:
patterning the projection beam with said patterning means; and
characterized by the steps of:
measuring at least one parameter of an image formed by the projection system, for a plurality of different settings of said radiation system and/or said projection system; and
calculating at least one coefficient, representative of aberration of said imaging system, on the basis of said at least one parameter measured at said plurality of settings.
Preferably, said plurality of different settings comprise different numerical aperture settings and/or sigma settings, illumination modes or telecentricity modes; furthermore, one may use various types and sizes of test structures on, for instance, one or more masks, to create different diffraction effects in the projection system. All such variation should be interpreted as falling within the meaning of the phrase xe2x80x9cdifferent illumination settingsxe2x80x9d as used in this text. The term xe2x80x9csigma ("sgr") settingxe2x80x9d refers to the radial extent of the intensity distribution in the beam at a pupil in the imaging system through which the radiation passes, normalized with respect to the maximum radius of the pupil. Thus, a sigma value of 1 represents an illumination intensity distribution with a radius at the pupil equal to the maximum radius of the pupil. The term xe2x80x9cillumination modexe2x80x9d denotes the spatial distribution of the radiation at the pupil, which may be, for example, disc-shaped, annular (which would be characterized by sigma inner and sigma outer settings), quadrupolar, dipolar, soft-multipolar (including some radiation flux in between the poles), etc. The term xe2x80x9ctelecentricity modesxe2x80x9d encompasses configuring the imaging system telecentrically and/or with varying degrees of non-telecentricity, for example by the use of prisms on top of a mask to tilt the illumination profile. These different settings can be selected conveniently in a lithographic projection apparatus.
The measured parameter can be one or more of: the position of best-focus of said image; the lateral position of said image; the deformation of said image; and other properties of exposing said image lithographically, such as line width and shape, and distance between adjacent structures.
Preferably, the plurality of different settings are selected such that the variation in the or each at least one measured parameter is substantially maximized. In this way the accuracy of the determined coefficient(s) can be improved.
Preferably, the plurality of different settings are selected such that the variation in said at least one measured parameter resulting from aberration represented by one or more of said coefficients is substantially zero, whilst the variation in said at least one parameter as the function of a coefficient that is to be determined, is non-zero. This technique enables different aberration coefficients, such as Zernike coefficients, to be obtained independently of each other.
The invention also provides a lithographic projection apparatus for projecting a patterned beam of radiation onto a substrate provided with a radiation-sensitive layer, the apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate;
a projection system for projecting the patterned beam onto a target portion of the substrate; and
illumination setting means for providing a plurality of different illumination settings of said radiation system and/or said projection system;
characterized by further comprising:
measuring means for measuring at least one parameter of a projected image formed by the projection system;
control means for selecting a plurality of different illumination settings at which said measuring means takes measurements; and
calculation means for calculating at least one coefficient, representative of aberration in said projection and/or radiation system, on the basis of said at least one parameter measured by said measuring means.
According to a further aspect of the invention there is provided a device manufacturing method comprising the steps of:
(a) providing a substrate that is at least partially covered by a layer of radiation sensitive material;
(b) providing a projection beam of radiation using a radiation system;
(c) using patterning means to endow the projection beam with a pattern in its cross-section;
(d) using a projection system to project the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, and characterized by the steps of:
measuring, prior to step (d), at least one parameter of an image formed by the projection system, for a plurality of different settings of said radiation system and/or said projection system;
calculating at least one coefficient, representative of aberration of said projection and/or radiation system, on the basis of said at least one parameter measured at said plurality of settings;
correcting for said aberration on the basis of said at least one calculated coefficient, to reduce aberration of an image projected by said projection system.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).